1. Field of the Invention
This invention relates to a magnetic recording disk, and more particularly to a rigid magnetic recording disk formed by applying a flexible magnetic sheet to a surface of a rigid disk base.
2. Description of the Prior Art
There has been known a rigid magnetic recording disk comprising a rigid disk base of aluminum (e.g., JIS A5086) and a magnetic layer formed on a surface of the rigid disk base. The rigid magnetic recording disk is manufactured by machining the rigid disk base, polishing the surface of the machined rigid disk base, and forming a magnetic layer on the surface of the polished rigid disk base by vapor deposition, spin-coating or the like.
Generally the surface of the magnetic recording disk is preferred to be as smooth as possible in order to minimize the space between the disk surface and the magnetic head for recording and/or reproduction and to effect high density recording and reproduction. However, in the case of the rigid magnetic recording disk having an aluminum disk base, it has been very difficult to obtain a surface having a centerline average height of not larger than 0.1 .mu.m. Further, since the disk base is rigid, it is difficult to continuously apply magnetic layers to disk bases, and manufacturing cost and steps of the disk are therefore increased.
Further, though the magnetic head must trace the magnetic layer of the disk while being spaced therefrom by a predetermined small distance during recording or reproduction, it is difficult, in the case of the rigid magnetic recording disk, to keep the small distance constant due to lack of flexibility of the disk. Further, in the case of the conventional rigid magnetic recording disk, if the magnetic head is brought into contact with the magnetic layer, surface fracture can be produced in the magnetic layer due to the rigid disk base, thereby shortening the service life of the disk. Accordingly, it is very difficult to reduce the distance between the magnetic layer and the magnetic head to further increase the recording density.
Further, the aluminum disk base itself is very expensive.
Thus, there has been proposed a magnetic recording disk comprising a rigid disk base 51 provided with a recess 54 on each side thereof and a floppy disk or a flexible disk 52 bearing a magnetic layer on one side thereof is applied to each side of the rigid disk base 51 over the recess 54 so that a space 53 is formed between the disk base 51 and each flexible disk 52 as shown in FIG. 4.
In the magnetic recording disk of this type, surface fracture of the magnetic layer is less apt to be produced if the magnetic head is accidentally brought into contact therewith or the magnetic head is brought into contact therewith in order to increase the recording density. Further, since the art of the flexible disk can be applied as it is, and the magnetic layer of the flexible disk which has a high durability and a smooth surface can be used as the magnetic layer, the problems inherent to the conventional rigid magnetic recording disks as described above can be avoided.
In the magnetic recording disk of this type, the manufacturing accuracy of the disk base need not be so high, and accordingly the disk base may be formed by injection molding in order to reduce the manufacturing cost. Further, by increasing the depth of the recesses in the disk base, the amount of material can be reduced, thereby further reducing the manufacturing cost. However, the magnetic recording disk is rotated at a high speed (e.g., 3600 rpm) during recording or reproduction, and it is required to keep constant the rotational speed of the disk in order to obtain high recording accuracy and high reading accuracy. In order to keep constant the rotational speed of the disk, it is preferred that the rotational inertia energy of the disk be as large as possible. That is, in order to reduce the manufacturing cost, the recess in the disk base is preferably deep, and on the other hand, in order to keep constant the rotational speed of the disk, the recess in the disk base is preferably shallow so that the mass of the disk base is increased.